Physiological, growth and root biochemical responses of Arbutus unedo and Quercus suber seedlings to UV radiation and water availability before and after aboveground biomass removal

Abstract

Models predict an increase in solar ultraviolet (UV) radiation, a decrease in precipitation and a higher frequency of disturbances (e.g. fires) in the near future in the Mediterranean basin. Thus, this study examined the effects of enhanced UV radiation (UV-A and UV-B) and diminished water supply on leaf morphology and physiology, plant growth, root reserves and resprouting capacity of two common Mediterranean species, before and after aerial biomass removal (simulating a disturbance, such as a severe fire). An outdoor experiment was conducted growing potted seedlings of strawberry tree (Arbutus unedo) and cork oak (Quercus suber) with enhanced UV-A + UV-B, enhanced UV-A or ambient UV, combined with optimal or reduced watering. Eight months after the start of the experiment, all the plant aerial biomass was removed and new shoots (resprouts) developed for a further 8 months under the UV and watering treatments. For both species, the responsiveness of seedlings (pre-disturbance) and resprouting plants (post-disturbance) to the treatments differed. Enhanced UV-B reduced the leaf relative water content (RWC) in well-watered A. unedo plants (seedlings and resprouts), decreasing also the amount of root phenols in well-watered resprouts of this species. In parallel, however, exposure to enhanced UV-B increased the photosynthetic rates of A. unedo resprouts, regardless of the irrigation level. Seedlings and resprouts of Q. suber responded to enhanced UV-A by hardening the leaves (increased leaf mass per area, LMA), although UV-A-supplemented seedlings allocated less carbon to produce leaves in relation to roots. Most UV effects were found only under optimal irrigation, suggesting a decrease in plant UV sensitivity under low water availability. In Q. suber, water shortage per se reduced leaf:total biomass and the concentration of sugar in roots. Despite some UV and watering effects on the parameters studied, the protective mechanisms displayed by both species were apparently effective since root starch reserves and plant resprouting capacity were not reduced by the treatments.